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How to mock methods with void return type?

I implemented an observer pattern but I can't mock it with Mockito because I don't know how.

And I tried to find an example on the Internet, but didn't succeed.

My class looks like this:

public class World {

    List<Listener> listeners;

    void addListener(Listener item) {

    void doAction(Action goal,Object obj) {
        setState("i received");
        setState("i finished");

    private string state;
    //setter getter state

public class WorldTest implements Listener {

    @Test public void word{
    World  w= mock(World.class);


interface Listener {
    void doAction();

The system are not triggered with mock. =( I want to show above mentioned system state. And make assertion according to them.

1 Answer

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by (32.3k points)

You can use any of the doThrow(),doAnswer(),doNothing(),doReturn() family of methods from Mockito framework to mock void methods(check out Mockito API docs)

For example,

Mockito.doThrow(new Exception()).when(instance).methodName();

or you may combine it with follow-up behavior,

Mockito.doThrow(new Exception()).doNothing().when(instance).methodName();

Presuming that you are looking at mocking the setter setState(String s) in the class World below is the code uses doAnswer method to mock the setState.

World  mockWorld = mock(World.class); 

doAnswer(new Answer<Void>() {

    public Void answer(InvocationOnMock invocation) {

      Object[] args = invocation.getArguments();

      System.out.println("called with arguments: " + Arrays.toString(args));

      return null;



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